A. Field of the Invention
The present invention relates to an electron gun for a color cathode-ray tube, and more particularly to a configuration of electrodes of an electron gun constituting a main electrostatic focusing lens.
B. Description of the Prior Art
A conventional color cathode-ray tube with an in-line type electron gun is shown in FIG. 1. A glass envelope 1 of the tube is composed of a front panel 2 and a funnel 3 connected to panel 2. A fluorescent screen which is coated with three color phosphors for developing a color image is disposed on the inner wall of panel 2. A shadow mask 4 for color selection is disposed inside envelope 1 adjacent panel 2 and spaced from the fluorescent screen.
An electron gun 5 is coaxially disposed inside the tubular neck portion of funnel 3 to generate and direct three electron beams (each beam representing a respective one of three primary colors) along coplanar convergent paths through shadow mask 4 to the fluorescent screen. More specifically, the electron beams, which are composed of thermions, are emitted from cathodes 6, 7 and 8 of electron gun 5, and pass through corresponding apertures in first and second grid electrodes 9 and 10. Then each of the electron beams is directed along a respective one of electron beam paths 11, 12, and 13 (shown in solid lines in FIG. 1) to panel 2. Each of cathodes 6, 7 and 8 and its corresponding apertures in first and second grid electrodes 9 and 10 has an axis parallel to the other on a common plane, and each axis is coincident with a respective one of electron beam paths 11, 12 and 13.
Referring to FIG. 1, a line Z--Z extending along the central electron beam path 12, i.e., the center of electron beam paths 11, 12 and 13, to panel 2 is called the "axial direction". Similarly, a line X--X, which is perpendicular to the axial direction and extending across a common plane including electron beam paths 11, 12 and 13, is called the "horizontal direction". A line Y--Y (not shown), which is perpendicular to the axial and horizontal directions, is called the "vertical direction."
Each of the three electron beams travels through second grid electrode 10 along a respective one of electron beam paths 11, 12 and 13 on a common plane and then through third and fourth grid electrodes 14 and 15. Third and fourth grid electrodes 14 and 15 constitute auxiliary focusing lens. Then, each of the electron beams travels through first and second accelerating/focusing electrodes 16 and 17 constituting the main focusing lens of the electron gun. In the main focusing lens, a potential of 25 KV.about.35 KV is applied to second accelerating/focusing electrode 17 and a potential of about 20%.about.30% of that applied to second electrode 17 is applied to first electrode 16.
Since the center portion of the main focusing lens is coaxial with the central electron beam path 12, the central beam of the three electron beams, which travels through the center portion of the main focusing lens, is focused to be thin and accelerated to travel straight along the axial direction to the fluorescent screen. However, since the outer portions of the main focusing lens are not coaxial with the central electron beam path 12, the two outer beams of the three electron beams which travel through the outer portions of the main focusing lens are not only focused to be thin, but also subjected to a converging effect toward the central electron beams. Thus, the three electron beams are converged onto shadow mask 4 in an overlapping fashion and then accelerated to reach the fluorescent screen to form colors thereon.
To scan electron beams on the fluorescent screen, an external magnetic deflection yoke 18 is externally provided adjacent glass envelope 1. The above operation for thinning electron beams by the main focusing lens is called "focusing" and the above operation for converging electron beams by the main focusing lens is called "static convergence."
FIG. 2 illustrates a partially cutaway perspective view of first and second accelerating/focusing electrodes 16 and 17 constituting the main focusing lens of the prior art electron gun of FIG. 1. First accelerating/focusing electrode 16 is composed of a non-cylindrical tube with one end open ("open end") and another end (opposite the open end) partially closed ("closed end"). More specifically, first electrode 16 includes an envelope 19 and a closed end face 20 on the closed end integral with envelope 19. Closed end face 20 includes three separate beam passage apertures 111, 112, and 113 which are axially parallel to one another. Each of beam passage apertures 111, 112 and 113 is surrounded by a respective one of cylindrical lips 121, 122 and 123. Each cylindrical lip is projected from closed end face 20 inwardly toward the open end of envelope 19.
Second accelerating/focusing electrode 17 has substantially the same configuration as first electrode 16 and is symmetrical to first electrode 16 with respect to the horizontal direction. Second accelerating/focusing electrode 17 includes an envelope 21 and a closed end face 22 integral with envelope 21. Closed end face 22 includes three electron beam passage apertures 131, 132, and 133. Each of beam passage apertures 131, 132 and 133 is surrounded by a respective one of cylindrical lips 141, 142 and 143. Each cylindrical lip is projected from closed end face 22 inwardly toward the open end of envelope 21.
The outer beam passage apertures 111 and 113 of first accelerating/focusing electrode 16 are spaced from the central beam passage aperture 112 at an equal first distance, i.e., the center to center distance, along the horizontal direction. This distance coincides with the distance between each of the outer electron beam paths 11 and 13 and the central electron beam path 12 (shown in FIG. 1). Likewise, the outer beam passage apertures 131 and 133 of second accelerating/focusing electrode 17 are spaced from the central beam passage aperture 132 at an equal second distance. The second distance is slightly greater than the first distance. Closed end faces 20 and 22 of respective first and second accelerating/focusing electrodes 16 and 17 face one other and are spaced from one another at a given distance "g".
In this prior art configuration, three separate main focusing lenses are provided, each lens for a respective one of the three electron beams. For example, three pairs of electron beam passage apertures, a first pair having the outer apertures 111 and 131, a second pair having the central apertures 112 and 132, and a third pair having the outer apertures 113 and 133, are provided in first and second accelerating/focusing electrodes 16 and 17. Each pair of electron beam passage apertures are surrounded by a respective one of a first pair of lips 121 and 141, a second pair of lips 122 and 142, and a third pair of lips 123 and 143. Each of the three separate main focusing lenses focuses a respective one of the three electron beams which travels through a respective pair of electron beam passage apertures.
Since the second distance corresponding to beam passage apertures 131, 132, and 133 of second accelerating/focusing electrode 17 is greater than the first distance corresponding to beam passage apertures 111, 112, and 113 of first accelerating/focusing electrode 16, the central main focusing lens, which includes a pair of the central beam passage apertures 112 and 132, is coaxial with respect to the axial direction, while the outer main focusing lenses, which include a pair of the outer beam passage apertures 111 and 131 and a pair of the outer beam passage apertures 113 and 133, are not coaxial with respect to the axial direction.
Accordingly, the central electron beam passing through the central main focusing lens is focused to be thin and accelerated to travel straight along the axial direction to the fluorescent screen, while the outer electron beams passing through the outer main focusing lenses are not only focused to be thin, but also subjected to a converging effect toward the central electron beam.
Generally, the resolution of a color cathode-ray tube is affected by the focusing characteristics of the electron gun therein. The focusing characteristics of a color cathode-ray tube are closely related to the aperture diameter of the main focusing lens of the electron gun. More specifically, the magnification and spherical aberration of the main focusing lens affect the focusing characteristics of a color cathode-ray tube. These factors depend strictly on the intensity of the focusing effect of the main focusing lens. That is, if the size or diameter of the electrostatic lens is increased by a ratio R, the second axial derivative of a potential inside the electrostatic lens is decreased by 1/R.sup.2.
As a result, a strength A of the lens is: A.about.1/R, and a spherical aberration C of the lens is: C.about.1/R.sup.3. Accordingly, spherical aberration C can be significantly reduced by the enlargement of the aperture diameter of the main focusing lens, and the magnification of the main focusing lens can be reduced by reducing the strength of the lens. Further, by the following relationship a small electron beam spot can be obtained and thus the focusing characteristics of the lens can be improved:
Diameter of electron beam spot determined by magnification of the main lens Dx=M.multidot.dx; and PA1 Diameter of electron beam spot, resulting from spherical aberration Dc=1/2M.multidot.C.multidot..theta..sup.3, PA1 Where M: magnification of the main focusing lens; PA1 C: coefficient of spherical aberration of the main focusing lens; PA1 dx: size of the imaged object; and PA1 .theta.: incident angle of the electron beam upon the main focusing lens.
However, in applying the above relationship to the conventional in-line type color cathode-ray tube, since the three main focusing lenses, each lens corresponding to an electron beam, are arranged on a common horizontal plane, the diameter of apertures 111, 112, 113, 131, 132 and 133 in first and second accelerating/focusing electrodes 16 and 17 constituting the main focusing lenses should be less than 1/3 of the inner diameter of the tubular neck portion of glass envelope 1 accommodating electron gun 5. The maximum permissible size of the diameter of the electron beam passage apertures is further restricted when the thickness and machining tolerances of the accelerating/focusing electrodes are considered.
Although the enlargement of the inner diameter of the neck portion to enlarge the diameter of the electron lens is conceivable, this increases deflection power. Further, the enlargement of the diameter of the beam passage apertures increases the center-to-center distance between the apertures, thus adversely affecting the converging-effect.
As a solution to this problem, in U.S. Pat. Nos. 4,599,534 (issued Jul. 8, 1986) and 4,626,738 (issued Dec. 2, 1986), attempts have been made to form the apertures in a separate member to maximize the aperture diameter within the physical constraint, instead of forming the apertures of the first and second acceleration/focusing electrodes directly in the closed end faces of the electrodes as discussed above. In the above patents, to enlarge the effective diameter of the main focusing lens and eliminate astigmatism by equalizing the horizontal and vertical focusing effects of the lens, an electrostatic field control electrode composed of an envelope and a separate electrode plate having elliptical polygonal apertures inside the envelope of the tube is employed.
Although the above approach may help reduce the number of components in the electrostatic field control electrode, to effectively control the deep penetration of electric field into the confronting electrodes in the horizontal and vertical directions, the dimensions of the elliptical or polygonal aperture have to be defined by polynomial or other complex expressions. Thus, the electrode design becomes complicated, and it becomes difficult to make design changes for the electron gun in response to changes in the screen size of the color cathode-ray tube and/or the type of the deflection yoke used therein.